Paper products are coated or varnished after printing to increase slip and to reduce the abrasion and friction. Such treatment results in an over print varnish which helps to reduce any negative impact that may arise to printed paper surfaces from further handling. The treatment is also used to improve the overall gloss of the paper.
Over print varnish compositions are especially applied in the manufacture of brochures, advertisement prints or packages produced with offset printing technology. The purpose of the over print varnish is to protect printed surfaces during processing after the print and upgrade its appearance due to the additional level of gloss.
Over print varnishes used to finish and condition printed papers may be oil- or water-based, and based on UV-crosslinking polymers, lacquers or waxes.
Since the development of water based inks in the 1970-ies water based over print varnishes have increasingly been applied after the printing process in an offset printing press. Water based over print varnishes are still commonly used until present.
U.S. Pat. No. 3,985,932 describes the use of waxes in water based paper coatings. According to the disclosure a paper coating additive can be obtained by using hard waxes, especially Fischer-Tropsch waxes, with a melting point above 80° C. and a particle size less than 50 μm. The waxes are suggested as a substitute for calcium stearate which has been commonly used as lubricating agent, and in order to improve the gloss and the brightness of the printed paper.
Also the installation engineering of offset printing machines developed over the years and was adapted to apply water based inks and over print varnishes. Through the change from two-roll-systems to wipe-off chamber-machines the spreading of the varnish became more homogeneous, the dosage accuracy of the varnish was improved and additional pigments could be applied (e.g. pearlescent or metallic pigments).
EP 0930161 B1 teaches the use of special anilox rolls to spread 10 to 30% silicone-, silicone wax- or wax-emulsions as over print varnish on paper, which provide a homogeneous and sufficient coating of the paper, show no mottling and reduce the formation of wax deposits.
The greatest disadvantages often associated with water based over print varnishes are the low heat-resistance, an inhomogeneous coating due to the oil-repellency of dye residuals on the surface as well as the insufficient adhesion between multiple layers.
Alternative products such as silicone oil-based emulsions are expensive, do not produce a reliable gloss and have a high deposition tendency which can block the applicator roll in the printing press, thereby making time consuming cleaning operations necessary.
It is therefore an aim of the present invention to overcome above insufficiencies and provide a water based wax formulation for surface curing of paper- and print-products with improved properties.
It was surprisingly found that water based wax formulations containing dialkylethers can fulfill these needs.
In the prior art dialkylethers, especially NACOL®-ether of Sasol (Dihexyl (C6), dioctyl (C8) and didecyl (C10)) are mentioned as a release agent, defoaming agent and an additive for a wide range of other applications (DE 102004056786).
DE 4237501A1 describes dialkyethers, in particular a dioctylether, as a replacement of mineral oil in metal surface treatment. The oil phase of the prepared 5% aqueous emulsions contained more than 50 wt.-% dialkylether and other components (for example fatty alcohol ethoxylates). The emulsions are used for anti-corrosion coatings for metals and have a good material compatibility with rubber gaskets.
EP 0635342 A1 discloses release emulsions for separating concrete from a mould using 2 to 3 weight parts of a dialkyether. An exemplary composition consists of 145 parts water, 35 parts palm kernel fatty acid-butylester, 13 parts di-hexylether, 9 parts of lauryl alcohol and 1 part of coconut fatty acid-diethanolamide.
WO 2010018017 A2 describes the use of long chain (C8 to C50) dialkyether in pulverulent material compositions (mortar, putty, glue joints etc.). The ether in the compositions acts as a dust-binder without adversely affecting other properties such as efficacy, stability and strength.
DE 19854531 A1 claims defoamer granules for solid detergents which contain in addition to silicones also dialkylethers (C6 to C22) as defoamers and mentions the advantageous dustbinding properties of dialkylethers and the ability to minimize the disadvantages of silicone coatings like stickiness.
DE 10133399 A1 describes compositions based on wax, which can be used as a basis for cosmetic products, but also for the impregnation and wetting of hygienic cloth paper. In addition to the wax component the formulations contain dialkyether as a constituent of the oil phase and less than 10 wt.-% of water. A preferred embodiment is composed of:                1-50 wt.-% of at least one dialkylether,        0.1-5 wt.-% of at least one active substance,        1-10 wt.-% of at least one oil,        0.1-10 wt.-% of at least one emulsifier,        5-90 wt.-% of wax components and        0-5 wt.-% water.        
DE 102005005176 A1 describes water-in-oil emulsions comprising solid dialkylethers and/or dialkycarbonates for cosmetic applications. Oil- and wax-components are mentioned as part of the liquid oil phase, which is the continuous phase of the emulsion. Preferred dialkylethers according to this reference are saturated, linear C12-C22-dialkyethers such as distearylether (C18H37—O—C18H37). The dialkylethers are used to increase the viscosity of the emulsion.
None of the cited prior art documents describe or suggest the use of dialkylethers in wax emulsions or dispersions to obtain a uniform paper coating.
It is therefore an objective of the present invention to provide a wax dispersion that is smoothly applicable to a paper surface such as by modern offset printing machines and when applied to the printed surface of a paper product results in an uniform paper coating that provides good gloss and protection to the surface.